U.S. Pat. No. 4,214,478 discloses an air-quantity measuring apparatus of the type referred to above wherein at least two films are deposited upon a carrier with at least one film serving as heating resistor for the carrier. The second film serves as measuring resistor of an anemometer configured for constant-temperature operation. For the heating resistor, a separate device for controlling its temperature is provided such that the carrier is adapted to be heated to the temperature of the second film by the heating resistor. The advantages of this known device are that the actual measuring resistor is assigned a heating resistor to provide protective heating which ensures a quick and accurate response of the device to changes in the quantity of the flowing air.
On the other hand, this known device is characterized by a highly complex circuit configuration using two different control systems. In addition, adapting the control time constants of the two control systems is problematical, so that in practice different time constants occur in the heating of the two resistors, causing heat flows which can not be ignored. Tests have shown that the geometry of the heating resistor relative to the measuring resistor on the substrate is not optimal.